Solid phase reactions between Fe thin films and Si-Ge layers on Si C.H. Yu a , Y.L. Chueh a , S.W. Lee a , S.L. Cheng a , L.J. Chen a, * , L.J. Chou a , L.W. Cheng b a Department of Materials Science and Engineering, National Tsing Hua University, 101, Section 2 Kuang-fu Road, 300 Hsinchu, Taiwan, ROC b United Microelectronics Corporation, Hsinchu, Taiwan, ROC Available online 9 April 2004 Abstract Solid phase reactions in Fe thin films on epi-Si 0.8 Ge 0.2 , poly-Si 0.7 Ge 0.3 , a-Si 0.8 Ge 0.2 , and a-Si 0.7 Ge 0.3 layers on silicon have been investigated. The as-deposited samples were in situ annealed in the ultrahigh vacuum chamber at 400 – 800 jC for 30 min. The island structure was found to cause the abrupt increase in the sheet resistance of the annealed Fe/SiGe samples at 700 – 800 jC. The formation of FeSi islands containing a small amount of Ge is attributed to the preferential reactions of Fe with Si to Ge. As the annealing temperature was raised to 800 jC, the Fe(Si 1Àx Ge x ) phase is the only phase found in the annealed Fe/epi-Si 0.8 Ge 0.2 and Fe/poly-Si 0.7 Ge 0.3 samples. On the other hand, at the annealing temperature above 700 jC, the h-Fe(Si 1Àx Ge x ) 2 phase was observed in the annealed Fe/a-Si 0.8 Ge 0.2 and Fe/a-Si 0.7 Ge 0.3 but the Fe(Si 1Àx Ge x ) is still the dominant phase. The results indicate that the formation of Fe disilicide was retarded by the presence of Ge atoms. D 2004 Elsevier B.V. All rights reserved. Keywords: h-FeSi 2 ; SiGe; Iron germanosilicide; Solid phase reaction 1. Introduction The semiconducting h-FeSi 2 has been considered to be a promising material for silicon based optoelectronic applica- tions in recent years [1]. The h-FeSi 2 has a direct band gap of about 0.87 eV [2–5], with a corresponding wavelength of 1.5 Am, which lies in the window of maximum transmission of the optical fibers. The growth of silicide layers on Si substrates has been achieved by solid phase epitaxy (SPE), reactive deposition epitaxy (RDE), molecular beam epitaxy (MBE), ion beam synthesis (IBS), and ion beam assisted deposition (IBAD) [6–13]. Fabrication of FeSi 2 precipitates in a silicon matrix has attracted much attention, owing to its higher photoluminescence than continuous iron disilicide layers. The IBS using Fe + implantation to embed iron FeSi 2 precipitates in a Si p–n junction [14] and the reactive MBE processes [15] are the more common approaches for grow- ing FeSi 2 precipitates. Recently, SiGe films have been implemented for appli- cations in electronic and optoelectronic devices such as modulation doped field effect transistors and heterojunction bipolar transistors. The main attributes are its band gap and lattice parameter can be modified by varying the Ge content in SiGe films, while maintaining compatibility with the widely used Si process technology. The interfacial reactions between the metal (Ni, Co, and Ti) thin films and SiGe films have been extensively investigated [16–18]. The growth of h-FeSi 2 on the Si 0.93 Ge 0.07 layer by the RDE process has also been reported [19]. The Fe-Si-Ge alloy phase exhibits exciting direct optical absorption near 0.83 eV in the optical transmission measurements, revealing a shift of the band gap with regard to that of h-FeSi 2 . Since the atomic radius of Ge is larger than that of Si, when some Ge atoms replace some Si atoms in the h-FeSi 2 lattice, a certain kind of lattice distortion will take place. As a result, the band structure of h-FeSi 2 changes which leads to a shift of the band gap. On the other hand, the investigation on influence of Ge content on the growth of h-FeSi 2 has been relatively scarce [19,20]. In the present study, an investigation on solid phase reac- tions between Fe films and crystalline as well as amorphous SiGe films with Ge concentrations ranging from 20% to 30% has been conducted. 2. Experimental procedures Four different SiGe films including epitaxial Si 0.8 Ge 0.2 (epi-Si 0.8 Ge 0.2 ), polycrystalline Si 0.7 Ge 0.3 (poly-Si 0.7 Ge 0.3 ), amorphous Si 0.7 Ge 0.3 (a-Si 0.7 Ge 0.3 ) and amorphous 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.069 * Corresponding author. Tel.: +886-3-571-0521; fax: +886-3-571-8328. E-mail address: ljchen@mse.nthu.edu.tw (L.J. Chen). www.elsevier.com/locate/tsf Thin Solid Films 461 (2004) 81 – 85